1. Field of the Invention
Oxide-dispersion-hardened superalloys based on nickel which, by virtue of their outstanding mechanical properties at high temperatures, find use in the construction of thermal machines. Preferred use is as blade material for gas turbines.
The invention relates to the improvement of the mechanical properties of oxide-dispersion-hardened nickel-based superalloys, it being necessary under all circumstances to achieve a structure having coarse, longitudinally-directed columnar crystals.
In the narrower sense the invention relates to a process for the production of longitudinally-directed coarse-grained columnar crystals in a workpiece consisting of an oxide-dispersion-hardened nickel-based superalloy having a Cr content of 15.5 to 25% by weight of Cr, starting from a warm-worked fine-grained structural status.
2. Discussion of Background
In operation, gas turbine blades are exposed to complex thermal and mechanical stresses. The creep stress at high temperatures plays a dominant role here. Therefore, in the case of all of the superalloys used, the aim is to obtain the structural status of longitudinally-directed coarse columnar crystals, since with this structure the creep resistance is at its maximum because of the greatly reduced grain boundary surface except in the main stress direction. This also applies for superalloys further strengthened by intercalation of oxidic dispersed phases. Said superalloys in general now show a recrystallization mechanism which deviates from the customary "usual" behavior as determined for metallic materials. This behavior is known as "secondary recrystallization" and is initiated at a specific, well-defined constant temperature which is determined only by the alloy composition, but not by the previous history. The latter determines not the level of the recrystallization temperature but the recrystallizability to coarse grain per se, i.e. the ability or inability of the material to be converted at all into the status of coarse longitudinally-directed columnar crystals.
In the case of certain oxide-dispersion-hardened nickel-based superalloys the phenomenon is now found that, despite prior optimum harmonic treatment, they display only a slight tendency or no tendency at all to "secondary recrystallization", i.e. to form coarse crystals, during the subsequent annealing at "recrystallization temperature". In other words: these alloys cannot be brought by any pretreatment (selection of the compacting and extrusion temperature, the extrusion conditions, the "proportion of cold-working" during thermomechanical shaping, etc.) into a condition in which the propulsion or the grain boundary energy of the fine-grained material is sufficient to guarantee coarse crystals, having a main dimension of at least a few mm, over the entire cross-section of the workpiece on final annealing. The material either does not recrystallize at all, or it recrystallizes as fine grains (particle diameter of a few .mu.m) or only partially as coarse grains (at the edges or at the start of rods). The workpiece which is produced from the fine-grained material and is in its final form or near-final form is thus unusable.
It was observed that this phenomenon, which greatly restricts the field of use for gas turbine blades subject to high stress, is apparently linked to the composition of the material. In addition to the Cr content, the C content and the content of elements (Ti, Zr, Ta, Hf) which form thermodynamically very stable carbides, even in small amounts and variations in amount, apparently play an important role. Despite taking various hypotheses as the starting point, it has not yet been possible to uncover clear, unambiguous and systematic relationships.
The following publication is cited in respect of the prior art:
C. P. Jongenburger, K. Lempenauer and E. Arzt, "The effect of heating rate in the Recrystallization behavior of high .gamma.', ODS Superalloys", Fall Meeting, USA, Oct. 1-5, 1989